Uncontrolled activation of the alternative pathway of complement is central to the pathogenesis of multiple kidney diseases. Factor H is the main circulating regulator of the alternative pathway. It contains two binding regions, and mutations in one of these regions (referred to as Short Consensus Repeat 19-20, or SCR 19- 20) are associated with atypical hemolytic uremic syndrome. Thus, SCR 19-20 is believed to mediate binding of factor H to endothelial cells. Factor H defects are also associated with other kidney diseases, including C3 glomerulopathy, IgA nephropathy, and lupus nephritis. Furthermore, a group of proteins that antagonize factor H, called the factor H related proteins (FHRs), are associated with kidney disease. However, we do not yet have a unified understanding of why the different genetic variants in factor H or the FHRs cause distinct ultrastructural patterns of glomerular injury. It is also not known why the kidney is so uniquely vulnerable to injury in patients with systemic factor H mutations. Thus, greater understanding is needed regarding how these proteins interact with each other and with the kidney. In the last funding period of this grant we discovered that a protein produced within the kidney, annexin A2, blocks the other binding region of factor H, SCR 6-8. Overexpression of annexin A2 causes complement activation throughout the kidney, demonstrating that SCR 6-8 is critical for controlling complement activation on kidney surfaces. Interestingly, many disease-associated variants of the FHRs contain reduplications of this binding region. Based on these findings, the central hypothesis of this grant is that SCR 6-8 is critical for controlling alternative pathway activation on the glomerular basement (GBM), and SCR 19-20 is critical for controlling activation on endothelial cells. Mutations or proteins that interfere with these binding regions predispose patients to C3G and aHUS, respectively. To test this hypothesis, the following specific aims will be pursued. Aim 1) Identify the molecular factors that control complement activation on renal surfaces. We will use in vitro systems to we will directly test whether SCR 6-8 mediates binding of factor H to the GBM and identify the binding ligands. Aim 2) Test whether the FHRs and annexin A2 cause complement dysregulation in the kidney. We will use animal models to test the hypothesis that the FHRs and annexin A2 cause complement activation on specific surfaces within the kidney. Aim 3) Develop novel therapies for blocking complement activation in the kidney. In this aim we will test whether new therapeutic strategies can specifically inhibit complement activation in the kidney while leaving other mechanisms of activation intact. This project is innovative because it provides a rational system for understanding how the numerous reported defects in factor H contribute to multiple different kidney diseases, and it provides an explanation for why the kidney is so uniquely susceptible to alternative pathway-mediated injury. The studies in this grant are significant because they test new treatment strategies for blocking complement activation specifically within the kidney.